ramips: fix reporting effective VLAN ID on MT7621 switches

On MT7621, the REG_ESW_VLAN_VTIM reads are undefined, causing swconfig
to always report `vid: 0` in swconfig show output.

Since a 4K VLAN table is used on this platform, the VLAN ID always
correponds to the actual VLAN table index so provide a specific MT7621
implementation of the get_vid callback which returns the table index
as VLAN ID.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>